Today's DC-DC converters (for example a boost- or a buck-converter, a boost-buck converter or a SEPIC) operate at a low conversion efficiency at small input currents. Traditional regulation loops, for example pulse width modulation (PWM) control or continuous conduction mode, are not suitable for small input currents and lead to low conversion efficiency. In particular for input currents in a range between several μA and about 100 mA, DC-DC converters according to the prior art lack stable operation and have high quiescent current consumption. For power sources having a high impedance characteristic, a DC-DC converter is needed which has an adjustable input current limit to charge up for example a storage capacitor or a battery from a low power source.